One flat knows engines and with opposed cylinders, which are equipped with an reticulated connecting rod assembly placed in a casing in two parts whose mating plane is located in the axis of the crankshaft and perpendicular to the plan which passes by the axis of the cylinders.
With this known design of the engines flat and with opposed cylinders, the pressures of explosion are transmitted, through the crankshaft, directly with the screw fasteners of the two half-casings. So very quickly, the casing becomes deformed and flees.
Moreover, with the known engine mentioned above, the crankshaft is heavier because it must comprise masses of balancing much more important. The engine itself is heavy, bulky, and with much of parts moving.